Overview
In the earliest stages of its existence, this incarnation of the Digital World was little more than a barren desert. As it grew alongside the Earth's communications network, however, it evolved and changed in dynamic ways, with more and more "layers" coming into existence over the top of the old ones. The lowest layer is the original desert plain, and the highest is the home of the four Digimon Sovereigns that rule the Digital World. In between lie many small "mini-verses," self-contained environments especially suited to the Digimon that live there, including (but not limited to) an area of clouds and clockwork; a warped, black-and-white town; an area of forests, lakes and rivers; and a 'world' entirely composed of water. Random packets of discarded data often roam the Digital World like tumbleweeds, and can be used to help repair and heal injured Digimon.
Although the Digital World has both night and day, there is no transitional period between the two, with darkness instantly sweeping across the world like a sheet. Visible in the sky no matter what plain it is viewed from is a pulsing globe of code and light, which represent the Earth. From it emanate "data streams," vast columns of energy which reach down into the Digital World and manipulate is digital matter, physical representations of acts performed on computers across the world. Should an individual be caught in a data stream, they will be swept up into it and deposited elsewhere, on any of the Digital World's potential planes. Between Earth and the Digital World is a warped region of inter dimensional space where reality is constantly in flux, and defined only by the perceptions of individuals passing through it.
Digimon are, in fact, not native to this Digital World, having been created by a group of teenage computer programmers in the late 1980s as an experiment into the development of artificial intelligence. When their project was shut down due to lack of funds, their primitive digital life forms accidentally found their way into the Digital World, where they began to grow and evolve. One of the programmers, Shibumi, observed this, and wrote an algorithm that allowed the Digimon to evolve beyond their original specifications and truly become individual life forms. Four Digimon in particular evolved into especially powerful forms and became the aforementioned "Digimon Sovereign."
On the human side of things, the world is 'average' and indistinguishable from standard earth, sharing similar levels of technology, languages and social matters. In fact for most humans of this world, digimon are nothing but fantasy, video game or card characters, a big entertainment franchise, so any mention of digimon on the 'human' side of things will mostly end with curious glances, or being pointed to the nearest game or manga store. This is due to efforts to keep the 'process' incident under wraps from the civilians to avoid panic.
Digimon sightings are rare, limited to digimon manifesting in the world in secluded places, and handled by 'tamers' and digimon living in hiding in the human world. Travel between the two realms is a bit tricky, either finding a hidden tunnel, leading to whatever layer it connects. Digimon can open 'tears' themselves, such gates are temporary, but are a bit easier to aim at a specific layer.
Regarding Integration.
The vines created portals on both sides of this world, with the majority of them located on different levels of the digital world. On the human side, thankfully, there are few and far between, and usually ended up as well hidden as the connections with the digital world itself, but some of them were found by a certain goverment agency in monitoring unsual activity, which until recently, limited to digimon stuff. In general, for the human population, there is no such a thing as the world tree, and the agency itself is neutral about it, after negotiations with digimon and deciding to 'see what happens'.
The Sovereigns at first were wary of these gates, and interacting with other worlds, but certain incidents taught them they can trust humans (and other creatures) and recently started to change their ruling about such visitors. They started to encourage fellow digimon to explore other realms, report what they find, and in general, simply try to create connections with other realms. Visitors are allowed, but might be received with joy, or wariness, depending on who meets said visitors, and might be under some slight surveilance now and then, just to be safe. There have been only a handful of digimon venturing to the tree itself, so it is not surprising not many might know what a Digimon is.